Bonding of composite materials

ABSTRACT

A method for surface preparation of composite substrates prior to adhesive bonding. A curable surface treatment layer is applied onto a curable, resin-based composite substrate, followed by co-curing. After co-curing, the composite substrate is fully cured but the surface treatment layer remains partially cured. The surface treatment layer may be a resin film or a removal peel ply composed of resin-impregnated fabric. After surface preparation, the composite substrate is provided with a chemically-active, bondable surface that can be adhesively bonded to another composite substrate to form a covalently-bonded structure.

This application claims the benefit of U.S. Provisional PatentApplication No. 62/074,266, filed Nov. 3, 2014, the disclosure of whichis incorporated by reference in its entirety.

BRIEF DESCRIPTON OF THE DRAWINGS

FIGS. 1A-1B illustrate a method of preparing a bondable surface on acomposite substrate using a peel ply, according to one embodiment of thepresent disclosure.

FIG. 2 schematically illustrates a composite substrate having a surfaceresin film formed thereon according to another embodiment of the presentdisclosure.

FIG. 3 illustrates adhesive bonding of two composite substrates aftersurface treatment.

FIG. 4 is the trace for a DSC measurement of a surface treatment resinaccording to one embodiment versus a standard prepreg resin.

DETAILED DESCRIPTION

Adhesive bonding has been conventionally used as a method for joiningcomposite structures, such as those used in the aerospace industry.Currently, adhesive bonding of composite structures is carried outpredominantly by one of three ways: (1) co-curing, (2) co-bonding, and(3) secondary bonding.

“Co-curing” involves joining uncured composite parts by simultaneouslycuring and bonding, wherein the composite parts are being cured togetherwith the adhesive, resulting in chemical bonding. However, it isdifficult to apply this technique to the bonding of uncured prepregs tofabricate large structural parts with complex shapes. Uncured compositematerials, e.g. prepregs, are tacky (i.e. sticky to the touch) and lackthe rigidity necessary to be self-supporting. As such, uncured compositematerials are difficult to handle. For example, it is difficult toassemble and bond uncured composite materials on tools with complexthree-dimensional shapes.

“Co-bonding” involves joining a pre-cured composite part to an uncuredcomposite part by adhesive bonding, wherein the adhesive and the uncuredcomposite part are being cured during bonding. The pre-cured compositeusually requires an additional surface preparation step prior toadhesive bonding.

“Secondary bonding” is the joining together of pre-cured composite partsby adhesive bonding, wherein only the adhesive is being cured. Thisbonding method typically requires surface preparation of each previouslycured composite part at the bonding surfaces.

Proper surface treatment for co-bonding and secondary bonding is aprerequisite to achieve the highest level of bond line integrity inadhesively bonded structures. Bond line integrity, generally, refers tothe overall quality and robustness of the bonded interface. Conventionalco-bonding and secondary bonding processes typically include a surfacetreatment of the composite structures pursuant to the manufacturer'sspecifications prior to adhesive bonding. Surface treatments include,but are not limited to grit blasting, sanding, peel ply, priming, etc.These surface treatment methods improve adhesion predominantly bymechanical roughening of the surface. The roughened surface allows forbetter adhesion due to mechanical interlocking at the bonding interface.Such co-bonding or secondary bonding of pre-cured composite structureshas a limitation in that the bonding mechanism occurs only throughmechanical interlocking with no formation of chemical bonds as inco-cure bonding. Such surface treatments, if performed improperly, couldbecome a source of bond failure during the use of the final bondedstructure. Furthermore, in the absence of chemical bond formation at theinterface of a composite bonded assembly, the assessment of bond linequality is critical to ensure that proper bonding has occurred.Unfortunately, assessment of bond line quality is often difficult andcurrent techniques known in the art to measure bond line quality are notwell suited to measure and evaluate all potential sources of weak bonds.

In the aerospace industry, adhesives are typically used in combinationwith mechanical fasteners (e.g. rivets, screws, and bolts) to safely andreliably secure structural materials. Rarely are structural adhesivesused as the sole mechanism for joining structural parts in an aircraft.Some of the benefits provided by adhesively bonded parts include lighterweight, reduced stress concentrations, durability, lower part count,etc. Despite these benefits, the use of adhesive bonding is limited due,in part, to the difficulty in assessing bond line integrity. Currently,a non-destructive method is not known to exist for measuring the bondstrength of joined parts. The only way to measure the strength of anadhesively bonded joint is to find the ultimate strength, which isobtained by breaking the bond. For obvious reasons, this type ofdestructive testing is not practical in an industrial manufacturingenvironment such as the assembly of an aircraft. Moreover, proof testinga large number of specimens to determine the average load capacity of anadhesive does not guarantee that each and every bonded structure willhave the expected bond strength.

In order to meet certain aviation certification requirements incountries such as the United States, structural redundancy of primarystructures is currently required. Current state-of-the-art bondingmethods are not able to satisfy those requirements. Currently, onlyco-cured structures are certified by the Federal Aviation Administration(FAA) in the United States for primary structures and are usedextensively in the aerospace industry. Thus, there remains a need for anadhesive bonding method or technology that can be used in amanufacturing environment as a method of creating reliable andhigh-strength chemical bonds while providing excellent reproducibilityof bond line quality. Furthermore, there remains a need for a bondingmethod that could satisfy the structural redundancy requirements (e.g.those set out by the FAA in the United States) without adding extramanufacturing steps.

A surface preparation method is disclosed herein that enables thecreation of a chemically-active composite surface, which is chemicallybondable to another substrate via the use of a resin-based adhesive.This bonding method creates a chemical bond between the compositesurface and the adhesive, resulting in a stronger bond betweensubstrates. Furthermore, this bonding process minimizes the effect ofcontamination on the bonding surfaces of the composite substrates. Inaddition, this bonding method can be practiced on an industrial scaleand does not require substantial change to the infrastructure currentlybeing used in the industry.

The bonding method disclosed herein allows for a way of achieving acertifiable bonding method by creating chemically reactive functionalgroups at the surface to be bonded, resulting in a co-cured structure.Consequently, the novel bonding method disclosed herein provides a wayof satisfying structural redundancy requirements such as those set outby the FAA in the United States without adding extra manufacturingsteps.

The aforementioned chemically active composite surface is created byusing a curable surface treatment layer that can be placed on afiber-reinforced resin substrate (or “composite substrate”). In oneembodiment, the curable surface treatment layer is a resin-rich peelply. FIGS. 1A-1B illustrate how a resin-rich peel ply is used to createa bondable surface with chemically-active functional groups. Referringto FIG. 1A, a curable peel ply 10 is first laminated onto an outermostsurface of an uncured or curable composite substrate 11. Theuncured/curable composite substrate is composed of reinforcement fibers11 a infused or impregnated with an uncured or curable matrix resin 11b, which contain one or more thermoset resins. As an example, thereinforcement fibers 11 a may be continuous unidirectional carbonfibers. The curable peel ply 10 is composed of a woven fabric 10 ainfused or impregnated with a curable matrix resin 10 b that isdifferent from the uncured/curable matrix resin 11 b of the compositesubstrate 11. The matrix resin of the peel ply 10 also contains one ormore thermoset resins; however, it is formulated so that the peel ply'sresin cures more slowly than the resin of the composite substrate 11. Asa result, the peel ply's resin is only partially cured when thecomposite substrate 11 is fully cured under the same curing conditions.Next, co-curing of the peel ply 10 and the composite substrate 11 iscarried out by heating at elevated temperature(s) for a pre-determinedtime period until the composite substrate 11 is fully cured, but thepeel ply 10 is only partially cured. As a result of co-curing, the peelply's matrix resin intermingles and reacts with the composite matrixresin at the interfacial region. The cure kinetics of the peel ply resinand of the substrate's matrix resin are controlled to obtain the desiredamount of intermingling between the peel ply resin matrix. Afterco-curing, the peel ply (including the fabric therein) is peeled off atthe fracture line 12 shown in FIG. 1A, leaving behind a remaining thinfilm of partially-cured resin 13 on the composite substrate 11 as shownin FIG. 1B. The fracture line 12 during peeling is at the fiber-resininterface, but not within the fabric. As a result, a rough, bondablesurface 13 a with chemically-active functional groups is formed (FIG.1B).

In another embodiment, the curable surface treatment layer is a curableresin film 20 (without any fabric embedded therein) as shown in FIG. 2.In this embodiment, the curable resin film 20 is formed on a compositesubstrate 21, which is composed of reinforcement fibers 21 a infused orimpregnated with an uncured or curable matrix resin 21 b, and theresulting structure is co-cured. As an example, the reinforcement fibers11 a may be continuous unidirectional carbon fibers. As in the case ofthe peel ply, the surface resin film is formulated so that it cures moreslowly than the resin of the composite substrate. As a result, when thecomposite substrate is fully cured, the surface resin film is onlypartially cured and the cured composite substrate is provided with abondable surface having chemically-active functional groups.

In the above embodiments, co-curing of the surface treatment layer (peelply/resin film) and composite substrate may be carried out at atemperature ranging from about room temperature (20° C-25° C.) to about375° F. (191° C.) for about 1 h to about 12 h at pressures ranging fromabout 0 psi to about 80 psi (or about 0 MPa to about 0.55 MPa).Moreover, co-curing may be achieved in an autoclave or by anout-of-autoclave process in which no external pressure is applied.

The first cured composite substrate 11 or 21 with the bondable surface,as discussed above, may be joined to a second composite substrate 30with a curable, resin-based adhesive film 31 sandwiched in between thesubstrates and in contact with the bondable surface 32 as shown in FIG.3. The resin-based adhesive film 31 is in an uncured or partially curedstate and possesses chemical functional groups that are capable ofreacting with the chemically-active functional groups on the bondablesurface 32 of the first substrate (11 or 21). During a subsequent heattreatment to affect bonding, these functional groups react with eachother to form chemical or covalent bonds.

The second composite substrate 30 may be a cured composite substratethat has been subjected to the same peel ply surface preparation asdescribed for the first composite substrate (11 or 21) so as to form acounterpart bondable surface with chemically-active functional groups.The joined composite substrates are then subjected to heat treatment atelevated temperature(s) to cure the adhesive, resulting in a covalentlybonded structure—this is referred to as secondary bonding. The adhesivefilm 31 may be applied to either or both of the bondable surfaces of thefirst and second composite substrates.

Alternatively, the bondable surface of the second composite substrate 30may be prepared by other known surface treatments such as sand blasting,grit blasting, dry peel ply surface preparation, etc. “Dry peel ply” isa dry, woven fabric (without resin), usually made out of nylon, glass,or polyester, which is applied to the bonding surface of the compositesubstrate followed by curing. After curing, the dry peel ply is removedto reveal a textured bonding surface.

In an alternative embodiment, the second composite substrate 30 is in anuncured state when it is joined to the first cured composite substrate(11 or 21). In such case, the uncured composite substrate 30 and thecurable adhesive film 31 are cured simultaneously in a subsequentheating step—this is referred to as co-bonding.

During co-bonding or secondary bonding of the composite substratesaccording to the methods disclosed herein, chemical or covalent bondsare formed between the reactive moieties present in the resin-basedadhesive and the chemically-reactive functional groups on the bondablesurface of the composite substrate, which are derived from the surfacetreatment layer (resin-rich peel ply/surface resin film). As a result,the covalently bonded structure has essentially no adhesive-compositeinterface. The presence of the chemically-active functional groups onthe bondable surface described herein optimizes the subsequent bondingprocess by increasing the bond strength between the bonded substratesand improving bonding reliability. Furthermore, the covalently bondedstructure is more resistant to contamination than bonded structuresprepared by conventional co-bonding or secondary bonding processes.

The terms “cure” and “curing” as used herein encompass polymerizingand/or cross-linking of a polymeric material brought about by mixing ofbased components, heating at elevated temperatures, exposure toultraviolet light and radiation. “Fully cured” as used herein refers to100% degree of cure. “Partially cured” as used herein refers to lessthan 100% degree of cure.

The degree of cure of the partially cured surface treatment layer afterco-curing with the composite substrate may be within the range of10%-75% of full cure, e.g. 25%-75% or 25%-50%. The partially curedsurface treatment layer (peel ply/resin film) containsunreacted/noncrosslinked functional groups, which is the source ofchemically-active functional groups for the bondable surface. The degreeof cure of a thermoset resin system can be determined by DifferentialScanning calorimetry (DSC). A thermoset resin system undergoes anirreversible chemical reaction during curing. As the components in theresin system cure, heat is evolved by the resin, which is monitored bythe DSC instrument. The heat of cure may be used to determine thepercent cure of the resin material. As an example, the following simplecalculation can provide this information:

% Cure=[ΔH _(uncured) −ΔH _(cured) ]/[ΔH _(uncured)]×100%

In the embodiments involving peel ply, the peel ply has a resin contentof at least 20% by weight based on the total weight of the peel ply,depending on the specific type of fabric being impregnated. In certainembodiments, the resin content is within the range of about 20% to about80% by weight, or about 20% to about 50% by weight. The fabric may becomposed of glass, nylon, or polyester fibers, although other types offabrics are contemplated herein. In one embodiment, the resin-rich peelply of the present disclosure contains, in weight percentages based onthe total weight of the peel ply: about 20% to about 80% ofthermosetting matrix resin, about 2% to about 20% curing agent(s), andabout 5% to about 40% of additional modifiers or filler additives.

In the embodiments described herein, the resin component of the surfacetreatment layer and that of the composite substrate are formed fromcurable resin compositions which include: one or more thermoset resins;at least one curing agent; and optionally, additives, modifiers, andfillers. The matrix resin of the composite substrate may also include aminor amount of thermoplastic materials, such as polyamide andpolyethersulfone, as tougheners.

Examples of suitable thermoset resins include, but are not limited to,epoxies, phenolics, cyanate esters, polyimides, bismaleimides,polyesters, polyurethane, benzoxazines (including polybenzoxazines),combinations thereof and precursors thereof.

Particularly suitable are multifunctional epoxy resins (or polyepoxides)having a plurality of epoxide functional groups per molecule. Thepolyepoxides may be saturated, unsaturated, cyclic, or acyclic,aliphatic, aromatic, or heterocyclic polyepoxide compounds. Examples ofsuitable polyepoxides include the polyglycidyl ethers, which areprepared by reaction of epichlorohydrin or epibromohydrin with apolyphenol in the presence of alkali. Suitable polyphenols thereforeare, for example, resorcinol, pyrocatechol, hydroquinone, bisphenol A(bis(4-hydroxyphenyl)-2,2-propane), bisphenol F(bis(4-hydroxyphenyl)-methane), fluorine 4,4′-dihydroxy benzophenone,bisphenol Z (4,4′-cyclohexylidene-bisphenol) and 1,5-hyroxynaphthalene.Other suitable polyphenols as the basis for the polyglycidyl ethers arethe known condensation products of phenol and formaldehyde oracetaldehyde of the novolac resin-type.

Examples of suitable epoxy resins include diglycidyl ethers of bisphenolA or bisphenol F, e.g. EPONTM 828 (liquid epoxy resin), D.E.R. 331,D.E.R. 661 (solid epoxy resins) available from Dow Chemical Co.;triglycidyl ethers of aminophenol, e.g. ARALDITE® MY 0510, MY 0500, MY0600, MY 0610 from Huntsman Corp. Additional examples includephenol-based novolac epoxy resins, commercially available as DEN 428,DEN 431, DEN 438, DEN 439, and DEN 485 from Dow Chemical Co.;cresol-based novolac epoxy resins commercially available as ECN 1235,ECN 1273, and ECN 1299 from Ciba-Geigy Corp.; hydrocarbon novolac epoxyresins commercially available as TACTIX ® 71756, TACTIX ®556, and TACTIX®756 from Huntsman Corp.

According to one embodiment, curing agents (or curatives) of the surfacetreatment layer are preferentially selected to allow for a slower curerate than that of the composite substrate's matrix resin. The curativesmay be selected from well-known curatives with reactivities that arewell established. For instance, curatives for epoxy resins in order ofincreasing curing rate are generally classified as:polymercaptan<polyamide<aliphatic polyamine<aromatic polyaminederivatives<tertiary amine boron trifluoride complex<acidanhydride<imidazole<aromatic polyamine<cyanoguanadine<phenol novolac.This list is only a guide and overlap within classifications exists.Curatives of the surface treatment layer are generally selected fromgroups that are listed towards the higher end of the reaction order,whereas the composite substrate's curatives may be generally selectedfrom groups towards the beginning of the reaction order.

Specific examples of curatives that may be used for the surfacetreatment layer and the composite substrate include, but are not limitedto, melamine and substituted melamine derivatives, aliphatic andaromatic primary amines, aliphatic and aromatic tertiary amines, borontrifluoride complexes, guanidines, dicyandiamide, bisureas (including2,4-Toluene bis-(dimethyl urea), commercially available as CA 150 fromCVC Thermoset Specialties), 4,4′-Methylene bis-(phenyl dimethylurea),e.g. CA 152 from CVC Thermoset Specialties), and4,4′-diaminodiphenylsulfone (4,4-DDS). One or more curing agents may becombined.

Table 1 provides some examples of curative pairing for epoxy-basedcomposite substrate (e.g. prepreg) and epoxy-based surface treatmentlayer to achieve different curing rates.

TABLE 1 Substrate (or Prepreg) Curative Surface Treatment Curative1,3-bis(4-aminophenoxy)benzene 4,4′-diaminodiphenylsulfone4,4′-diaminodiphenylsulfone melamine 4,4′-diaminodiphenylsulfone,4,4-′diaminodiphenylsulfone dicyanoguanadine4,4′-diaminodiphenylsulfone, melamine dicyanoguanadine3,3′-diaminodiphenylsulfone melamine 1,3-bis(4-aminophenoxy)benzene(3-(4-aminobenzoyl)oxyphenyl)-4- aminobenzoate3,3′-diaminodiphenylsulfone 4,4′-diaminodiphenylsulfone Bis-aniline M4,4′-diaminodiphenylsulfone 4,4′-diaminodiphenylsulfone(3-(4-aminobenzoyl)oxyphenyl)-4- aminobenzoate Bis-aniline P3,3′-diaminodiphenylsulfone 3,3′-diaminodiphenylsulfone, melamineisophthalic dihydrazide

According to another embodiment, the thermoset resin composition of thesurface treatment layer contains one or more cure inhibitors that areable to slow the rate of reaction between the thermoset resins andcuratives. Thus, the surface treatment layer may contain the samethermoset resins and curatives as those in the composite substrate, butwill cure at a slower rate due to the presence of the inhibitors. Forthe purposes of the present disclosure, any inhibitor which slows therate of reaction between the thermoset resin and the curative may beused.

For epoxy-based compositions, examples of suitable cure inhibitorsinclude, but are not limited to, boric acid, trifluoroborane, andderivatives thereof such as alkyl borate, alkyl borane,trimethoxyboroxine and organic acids having a pKa from 1 to 3 such asmaleic acid, salicyclic acid, oxalic acid and mixtures thereof. Otherinhibitors include metal oxides, metal hydroxides, and alkoxides ofmetal, where the metal is zinc, tin, titanium, cobalt, manganese, iron,silicon, boron, or aluminum. When such inhibitor is used, the amount ofinhibitor may be up to about 15 parts per hundred parts of resin or PHR,for example, about 1 to about 5 PHR, in a resin composition. “PHR” isbased on the total weight of all resins in the resin composition.

In another embodiment, the composite substrate's matrix resin maycontain one or more additives, accelerators, or catalysts that functionto increase the rate of reaction between the thermoset resin and thecurative therein.

Catalysts useful for the purposes disclosed herein are those catalystswhich catalyze the reaction of a thermoset resin with a curing agent.For epoxy resins, examples of suitable catalysts are compoundscontaining amine, phosphine, heterocyclic nitrogen, ammonium,phosphonium, arsenium or sulfonium moieties. Suitable catalysts areheterocyclic nitrogen-containing and amine-containing compounds.Suitable heterocyclic nitrogen-containing and amine-containing compoundswhich may be used herein include, for example, imidazoles,imidazolidines, imidazolines, benzimidazoles, oxazoles, pyrroles,thiazoles, pyridines, pyrazines, morpholines, pyridazines, pyrimidines,pyrrolidines, pyrazoles, quinoxalines, quinazolines, phthalozines,quinolines, purines, indazoles, indoles, indolazines, phenazines,phenarsazines, phenothiazines, pyrrolines, indolines, piperidines,piperazines, combinations thereof and the like. When such catalysts areused, the amount of catalyst(s) may be up to 15 parts per hundred partsof resin or PHR, for example, about 1 to about 5 PHR, in a resincomposition.

Inorganic fillers in particulate form (e.g. powder) may also be added tothe resin composition of the surface treatment layer/composite substrateas a rheology modifying component to control the flow of the resincomposition and to prevent agglomeration therein. Suitable inorganicfillers include, but are not limited to, fumed silica, talc, mica,calcium carbonate, alumina, ground or precipitated chalks, quartzpowder, zinc oxide, calcium oxide, and titanium dioxide. If present, theamount of fillers in the resin composition may be from about 0.5% toabout 40% by weight, or about 1 to about 10% by weight, or about 1 toabout 5% by weight, based on the total weight of the resin composition.

In the embodiments that use resin-rich peel ply for surface treatment,the peel ply may be formed by coating the resin composition onto thewoven fabric so as to completely impregnate the yarns in the fabricusing conventional solvent or hot-melt coating processes. The wet peelply is then allowed to dry to reduce the volatile content, preferably,to less than 2% by weight. Drying may be done by air drying at roomtemperature overnight followed by oven drying at about 140° F. to about170° F., or by oven drying at elevated temperature as necessary toreduce the drying time. Subsequently, the dried peel ply may beprotected by applying removable release papers or synthetic films (e.g.polyester films) on opposite sides. Such release papers or syntheticfilms are to be removed prior to using the peel ply for surfacetreatment.

In the embodiments that use surface resin film for surface treatment,the resin film may be formed by coating a resin composition onto aremovable carrier, e.g. release paper, using conventional film coatingprocesses. The wet resin film is then allowed to dry. Subsequently, theresin film is placed onto a surface of a composite substrate, and thecarrier is removed.

Composite Substrates

Composite substrates in this context refer to fiber-reinforced resincomposites, including prepregs or prepreg layups (such as those used formaking aerospace composite structures). The term “prepreg” as usedherein refers to a layer of fibrous material (e.g. unidirectional towsor tape, nonwoven mat, or fabric ply) that has been impregnated with acurable matrix resin. The matrix resin in the composite substrates maybe in an uncured or partially cured state. The fiber reinforcementmaterial may be in the form of a woven or nonwoven fabric ply, orcontinuous unidirectional fibers. “Unidirectional fibers” as used hereinrefers to a layer of reinforcement fibers that are aligned in the samedirection. The term “prepreg layup” as used herein refers to a pluralityof prepreg plies that have been laid up in a stacking arrangement. Asexample, the number of prepreg plies may be 2 to100 plies, or 10 to 50plies.

The layup of prepreg plies may be done manually or by an automatedprocess such as Automated Tape Laying (ATL). The prepreg plies withinthe layup may be positioned in a selected orientation with respect toone another. For example, prepreg layups may comprise prepreg plieshaving unidirectional fiber architectures, with the fibers oriented at aselected angle θ, e.g. 0°, 45°, or 90°, with respect to the largestdimension of the layup, such as the length. It should be furtherunderstood that, in certain embodiments, the prepregs may have anycombination of fiber architectures, such as unidirectionally alignedfibers, multi-directional fibers, and woven fabrics.

Prepregs may be manufactured by infusing or impregnating continuousfibers or woven fabric with a matrix resin system, creating a pliableand tacky sheet of material. This is often referred to as a prepreggingprocess. The precise specification of the fibers, their orientation andthe formulation of the resin matrix can be specified to achieve theoptimum performance for the intended use of the prepregs. The volume offibers per square meter can also be specified according to requirements.

The term “impregnate” refers to the introduction of a curable matrixresin material to reinforcement fibers so as to partially or fullyencapsulate the fibers with the resin. The matrix resin for makingprepregs may take the form of resin films or liquids. Moreover, thematrix resin is in a curable or uncured state prior to bonding.Impregnation may be facilitated by the application of heat and/orpressure.

As an example, the impregnating method may include:

-   -   (1) Continuously moving a layer of fibers (e.g., in the form of        unidirectional fibers or a fabric web) through a (heated) bath        of molten impregnating matrix resin composition to fully or        substantially fully wet out the fibers; or    -   (2) Pressing top and bottom resin films against a layer of        fibers (e.g., in the form of continuous, unidirectional fibers        arranged in parallel or a fabric ply).

The reinforcement fibers in the composite substrates (e.g. prepregs) maytake the form of chopped fibers, continuous fibers, filaments, tows,bundles, sheets, plies, and combinations thereof. Continuous fibers mayfurther adopt any of unidirectional (aligned in one direction),multi-directional (aligned in different directions), non-woven, woven,knitted, stitched, wound, and braided configurations, as well as swirlmat, felt mat, and chopped mat structures. Woven fiber structures maycomprise a plurality of woven tows, each tow composed of a plurality offilaments, e.g. thousands of filaments. In further embodiments, the towsmay be held in position by cross-tow stitches, weft-insertion knittingstitches, or a small amount of resin binder, such as a thermoplasticresin.

The fiber materials include, but are not limited to, glass (includingElectrical or E-glass), carbon (includinggraphite), aramid, polyamide,high-modulus polyethylene (PE), polyester, poly-p-phenylene-benzoxazole(PBO), boron, quartz, basalt, ceramic, and combinations thereof.

For the fabrication of high-strength composite materials, such as thosefor aerospace and automative applications, it is preferred that thereinforcing fibers have a tensile strength of greater than 3500 MPa.

Generally, the matrix resin of the composite substrates/prepregs issimilar to that of the surface treatment layer as described above.

Adhesive

The adhesive for bonding composite substrates is a curable compositionsuitable for co-curing with uncured or curable composite substrates. Thecurable adhesive composition may comprise one or more thermoset resins,curing agent(s) and/or catalyst(s), and optionally, toughening agents,fillers, flow control agents, dyes, etc. The thermoset resins include,but are not limited to, epoxy, unsaturated polyester resin,bismaleimide, polyimide, cyanate ester, phenolic, etc.

The epoxy resins that may be used for the curable adhesive compositioninclude multifunctional epoxy resins having a plurality of epoxy groupsper molecule, such as those disclosed for the matrix resin of the peelply and composite substrate.

The curing agents may include, for example, guanidines (includingsubstituted guanidines), ureas (including substituted ureas), melamineresins, guanamine, amines (including primary and secondary amines,aliphatic and aromatic amines), amides, anhydrides, and mixturesthereof. Particularly suitable are latent amine-based curing agents,which can be activated at a temperature greater than 160° F. (71° C.),or greater than 200° F., e.g. 350° F. Examples of suitable latentamine-based curing agents include dicyandiamide (DICY), guanamine,guanidine, aminoguanidine, and derivatives thereof. A particularlysuitable latent amine-based curing agent is dicyandiamide (DICY).

A curing accelerator may be used in conjunction with the latentamine-based curing agent to promote the curing reaction between theepoxy resins and the amine-based curing agent. Suitable curingaccelerators may include alkyl and aryl substituted ureas (includingaromatic or alicyclic dimethyl urea); bisureas based on toluenediamineor methylene dianiline. An example of bisurea is 2,4-toluenebis(dimethyl urea). As an example, dicyandiamide may be used incombination with a substituted bisurea as a curing accelerator.

Toughening agents may include thermoplastic or elastomeric polymers, andpolymeric particles such as core-shell rubber (CSR) particles. Suitablethermoplastic polymers include polyarylsulphones with or withoutreactive functional groups. An example of polyarylsulphone withfunctional groups include, e.g. polyethersulfone-polyetherethersulfone(PES-PEES) copolymer with terminal amine functional groups. Suitableelastomeric polymers include carboxyl-terminated butadiene nitrilepolymer (CTBN) and amine-terminated butadiene acrylonitrile (ATBN)elastomer. Examples of CSR particles include those commerciallyavailable under the trademark Kane Ace®, such as MX 120, MX 125, and MX156 (all containing 25 wt.% CSR particles dispersed in liquid BisphenolA epoxy).

Inorganic fillers may be in particulate form, e.g. powder, flakes, andmay be selected from fumed silica quartz powder, alumina, mica, talc andclay (e.g., kaolin).

EXAMPLES

The following Examples are provided to illustrate certain aspects of thepresent disclosure.

Example 1

This example demonstrates the effectiveness of a surface treatment basedon the concept of controlled curing kinetics.

A surface treatment film was formed by preparing a resin formulationcontaining, in parts by weight: 50 parts Dicyclopentadiene-containingnovolac epoxy resin; 80 parts diglycidyl ether of bis-phenol A; 10 partspara-amino phenol epoxy resin; 10 parts poly(ether)sulfone; 39 parts4,4′-diaminodiphenylsulfone; and 2 parts fumed silica.

The resin mixture was mixed using a hot-melt process followed by coatingthe resin mixture as an unsupported film at 0.054 psf (pounds per squarefoot). The resin film was manually laid up with 10 plies of prepregmaterial such that the resin film is the topmost layer. The prepregmaterial was composed of glass fibers impregnated with an epoxy-basedmatrix resin containing elastomer modified bis-A epoxy resins, novolacmodified epoxy resin, dicyanoguanadine, and1,1′-4(methyl-m-phenylene)bis(3,3′-dimethylurea). The uncured laminatewith the resin film was then cured by heating at 250° F. for 3 hours at80 psi. After cure, the cured composite was provided with a bondablesurface. The cured composite laminate was removed from the tool andjoined with another similarly prepared, cured composite laminate, whichwas subjected to the same surface treatment. No adhesive was used forthe bonding step and only the surface treatment functional groups wereavailable for bonding. The joined article was then heated at 350° F. for90 min. at 80 psi.

Example 2

The following example shows the effect of a surface treatment that doesnot contain a slow cure surface treatment film for comparison.

A surface treatment film was formed by preparing a resin formulationcontaining, in parts by weight: 50 parts Dicyclopentadiene-containingnovolac epoxy resin; 80 parts diglycidyl ether of bis-phenol A; 10 partspara-amino phenol epoxy resin; 10 parts poly(ether)sulfone; 29 parts4,4′-diaminodiphenylsulfone; 2 parts dicyandiamide; and 2 parts fumedsilica.

The resin mixture was mixed using a hot-melt process followed by coatingthe resin mixture as an unsupported film at 0.054 psf. The resin filmwas manually laid up with 10 plies of prepreg material such that theresin film is the topmost layer. The prepreg material was the same asthat described in Example 1. The uncured laminate was then cured byheating at 250° F. for 3 hours at 80 psi. After cure, the curedcomposite was provided with a bondable surface. The cured composite wasremoved from the tool and joined with another similarly prepared, curedcomposite laminate, which contained the same bondable surface. Noadhesive was used for the bonding step and only the surface treatmentfunctional groups were available for bonding. The joined article wasthen heated at 350° F. for 90 min. at 80 psi.

Example 3

The following example demonstrates a surface treatment which involvedthe use of a removable peel ply to improve surface roughness andfacilitate bonding.

A surface treatment layer was formed by preparing a resin formulationcontaining, in parts by weight: 50 parts Dicyclopentadiene-containingnovolac epoxy resin; 80 parts diglycidyl ether of bis-phenol A; 10 partspara-amino phenol epoxy resin; 10 parts poly(ether)sulfone; 19 parts4,4′-diaminodiphenylsulfone; and 2 parts fumed silica.

The resin mixture was mixed using a hot-melt process followed by coatingthe resin mixture onto a polyester-based fabric from Porcher Industries(Porcher 8115) to impregnate the fabric, and allowing theresin-impregnated fabric to dry, thereby forming a peel ply. The peelply was manually laid up with 10 plies of prepreg material such that thepeel ply is the topmost layer. The prepreg material is composed ofcarbon fibers impregnated with an epoxy-based matrix resin containing atetra-functional epoxy resin based on methylene dianiline, atrifunctional epoxy resin based on meta-aminophenol, polyether sulfone,3,3′-diamino-diphenylsulfone, and isophthalic dihydrazide (anaccelerator). The uncured composite laminate was then cured by heatingat 350° F. for 3 hours at 80 psi. After cure, the cured composite wasremoved from the tool, the peel ply removed, and adhesively joined withanother similarly prepared, cured composite laminate, which wassubjected to the same surface treatment with the peel ply. The adhesiveused was FM 309-1 (available from Cytec Engineered Materials). Thejoined article was then heated at 350° F. for 90 min. at 40 psi toachieve cure.

Mechanical Properties of Bonded Structures and Characterization

The mechanical performance of the bonded structures produced in Examples1-3 was determined by a G_(1c) fracture toughness test done inaccordance to ASTM D5528. The G_(1c) results are shown in TABLE 2.

TABLE 2 Fracture Toughness Example 1 Example 2 Example 3 G_(1c)(Joules/m²) 1802 116 1211

TABLE 2 shows the advantages of the surface treatment of the presentdisclosure by demonstrating that improved bond strength was achievedcompared to a surface treatment in which the curative caused full cureof the surface treatment resin.

Thermal Characterization

The rate of cure of the surface treatment layer compared to the rate ofcure of an underlying prepreg material can easily be assessed bydifferential scanning calorimetry (DSC). FIG. 4 shows the DSC profilefor the prepreg material and the surface treatment layer disclosed inExample 3. As can be seen from FIG. 4, the onset cure temperature of thesurface treatment resin is higher than that of the prepreg resin. Inthis particular example, the prepreg material began to undergo cure andconsumption of reactive epoxy functional groups at a rate that washigher than that of the surface treatment layer. Thus, following fullcuring of the prepreg material, the surface treatment resin was in apartially cured stated and contained unreacted functional groups.

What is claimed is:
 1. A method for surface preparation prior toadhesive bonding comprising: (a) providing a composite substratecomprising reinforcing fibers impregnated with a first curable matrixresin; (b) applying a surface treatment layer onto a surface of thecomposite substrate, said surface treatment layer comprising a secondcurable matrix resin different from the first matrix resin; (c)co-curing the composite substrate and the surface treatment layer untilthe composite substrate is fully cured but the surface treatment layerremains partially cured, wherein the second matrix resin is formulatedto cure at a slower rate than the first matrix resin, and afterco-curing (c), the surface treatment layer provides a bondable surfacewith chemically-active functional groups.
 2. The method of claim 1,wherein the surface treatment layer is a resin film which does notcomprise a fabric or reinforcement fibers embedded therein.
 3. Themethod of claim 1, wherein the surface treatment layer comprises a wovenfabric infused with the second curable matrix resin, and after co-curing(c), the surface treatment layer is removed from the compositesubstrate's surface, leaving a thin film of partially cured matrix resinon the composite substrate's surface, said thin film providing aroughened, bondable surface with chemically-active functional groups. 4.The method of claim 1, wherein the first and second curable matrixresins comprise one or more multifunctional epoxy resins.
 5. The methodof claim 1, wherein the first and second matrix resins comprisedifferent curing agents that are selected to affect curing at differentrates.
 6. The method of claim 5, wherein the first and second matrixresins comprise one or more epoxy resins, the curing agents for thefirst and second matrix resins are selected from the group consistingof: melamine and substituted melamine derivatives, polymercaptan,polyamide, aliphatic polyamine, aromatic polyamine derivatives, tertiaryamine boron trifluoride complex, acid anhydride, imidazoles, aromaticpolyamine, cyanoguanadine, phenol novolac, and the curing agents for thefirst and second curable matrix resin are selected to enable the firstmatrix resin to cure at a faster rate relative to that of the secondmatrix resin.
 7. The method of claim 6, wherein the first matrix resincomprises 1,3-bis(4-aminophenoxy)benzene, and the second matrix resincomprises 4,4′-diaminodiphenylsulfone or(3-(4-aminobenzoyl)oxyphenyl)-4-aminobenzoate, as curing agents.
 8. Themethod of claim 6, wherein the first matrix resin comprises4,4′-diaminodiphenylsulfone or 3,3′-diaminodiphenylsulfone, and thesecond matrix resin comprises melamine, as curing agents.
 9. The methodof claim 6, wherein the first matrix resin comprises the combination of4,4′-diaminodiphenylsulfone and dicyanoguanadine, and the second matrixresin comprises 4,4′-diaminodiphenylsulfone or melamine, as curingagents.
 10. The method of claim 6, wherein the first matrix resincomprises 3,3′-diaminodiphenylsulfone, and the second matrix resincomprises 4,4′-diaminodiphenylsulfone, as curing agents.
 11. The methodof claim 6, wherein the first matrix resin comprises Bis-aniline M, andthe second matrix resin comprises 4,4′-diaminodiphenylsulfone, as curingagents.
 12. The method of claim 6, wherein the first matrix resincomprises 4,4′-diaminodiphenylsulfone, and the second matrix resincomprises (3-(4-aminobenzoyl)oxyphenyl)-4-aminobenzoate, as curingagents.
 13. The method of claim 6, wherein the first matrix resincomprises Bis-aniline P, and the second matrix resin comprises3,3′-diaminodiphenylsulfone, as curing agents.
 14. The method of claim6, wherein the first matrix resin comprises the combination of3,3′-diaminodiphenylsulfone and isophthalic dihydrazide, and the secondmatrix resin comprises melamine, as curing agents.
 15. The method ofclaim 1, wherein the second curable matrix resin comprises one or morethermoset resins, a curing agent, and an inhibitor which can slow therate of reaction between the one or more thermoset resins and the curingagent in the second matrix resin.
 16. The method of claim 15, whereinsaid inhibitor is selected from the group consisting of: boric acid;trifluoroborane; alkyl borate; alkyl borane; trimethoxyboroxine; organicacids having a pKa from 1 to 3, including maleic acid, salicyclic acid,oxalic acid; metal oxides, metal hydroxides, and alkoxides of metal,where the metal is selected from: zinc, tin, titanium, cobalt,manganese, iron, silicon, boron, or aluminum; and combinations thereof.17. The method of claim 1, wherein the first curable matrix resincomprises one or more thermoset resins, a curing agent, and anaccelerator which can increase the rate of reaction between the one ormore thermoset resins and the curing agent.
 18. The method of claim 17,wherein the first matrix resin comprises one or more epoxy resins andthe accelerator is selected from compounds containing amine, phosphine,heterocyclic nitrogen, ammonium, phosphonium, arsenium or sulfoniummoieties.
 19. The method of claim 17, wherein the first matrix resincomprises one or more epoxy resins and the accelerator is selected fromthe group consisting of: benzimidazoles, imidazoles, imidazolidines,imidazolines, oxazoles, pyrroles, thiazoles, pyridines, pyrazines,morpholines, pyridazines, pyrimidines, pyrrolidines, pyrazoles,quinoxalines, quinazolines, phthalozines, quinolines, purines,indazoles, indoles, indolazines, phenazines, phenarsazines,phenothiazines, pyrrolines, indolines, piperidines, piperazines, andcombinations thereof.
 20. A cured composite substrate having a bondablesurface with chemically-active functional groups produced by the methodof claim
 1. 21. A bonding method comprising: (a) providing a firstcomposite substrate comprising reinforcing fibers impregnated with afirst curable matrix resin; (b) applying a removable, resin-rich peelply onto a surface of the first composite substrate, said peel plycomprising a woven fabric infused or impregnated with a second curablematrix resin, which is formulated to cure at a slower rate than thefirst curable matrix resin; (c) co-curing the first composite substrateand the peel ply until the first composite substrate is fully cured butthe second matrix resin in the peel ply remains partially cured; (d)removing the peel ply from the first composite substrate's surface,leaving a thin film of partially cured second matrix resin on the firstcomposite substrate's surface, said thin film providing a roughened,bondable surface with chemically-active functional groups; (e) joiningthe cured, first composite substrate to a second composite substratewith a curable adhesive film in between the composite substrates,wherein the curable adhesive film comprises chemically-active functionalgroups capable of reacting with the chemically-active functional groupson the bondable surface of the first composite substrate; and (f) curingthe adhesive film to form a covalently bonded structure.
 22. The bondingmethod of claim 21, wherein the second composite substrate is curedprior to being joined to the cured, first composite substrate.
 23. Thebonding method according to claim 21, wherein the second compositesubstrate is uncured or partially cured prior to being joined to thefirst composite substrate, and during curing at (f), the adhesive filmand the second composite substrate are cured simultaneously.
 24. Thebonding method of claim 21, wherein the first and second curable matrixresins comprise different curing agents that are selected to affectcuring at different rates.
 25. A bonding method comprising: (a)providing a first composite substrate comprising reinforcing fibersinfused or impregnated with a first curable matrix resin; (b) applying aresin film onto a surface of the first composite substrate, said resinfilm is formed from a second curable matrix resin, which is formulatedto cure at a slower rate than the first curable matrix resin; (c)co-curing the first composite substrate and the resin film until thefirst composite substrate is fully cured but the resin film remainspartially cured, thereby providing a bondable surface withchemically-active functional groups; (e) joining the cured, firstcomposite substrate to a second composite substrate with a curableadhesive film in between the composite substrates, wherein the curableadhesive film comprises chemically-active functional groups capable ofreacting with the chemically-active functional groups on the bondablesurface of the first composite substrate; and (f) curing the adhesivefilm to form a covalently bonded structure.
 26. The bonding method ofclaim 25, wherein the second composite substrate is cured prior to beingjoined to the cured, first composite substrate.
 27. The bonding methodof claim 25, wherein the second composite substrate is uncured orpartially cured prior to being joined to the first composite substrate,and during curing at (f), the adhesive film and the second compositesubstrate are cured simultaneously.
 28. The bonding method of claim 25,wherein the first and second curable matrix resins comprise differentcuring agents that are selected to affect curing at different rates.